On 5/20/26 10:05, Christian König wrote: > On 5/20/26 08:50, Dmitry Osipenko wrote: >> On 5/19/26 11:27, Christian König wrote: >>> On 5/19/26 10:22, Deepanshu Kartikey wrote: >>>> virtio_gpu_cursor_plane_update() and virtio_gpu_resource_flush() lock >>>> the framebuffer BO's dma_resv via virtio_gpu_array_lock_resv() and >>>> ignore its return value. The function can fail with -EINTR from >>>> dma_resv_lock_interruptible() (signal during lock wait) or with >>>> -ENOMEM from dma_resv_reserve_fences() (fence slot allocation), >>>> leaving the resv lock not held. The queue path then walks the object >>>> array and calls dma_resv_add_fence(), which requires the lock held; >>>> with lockdep enabled this trips dma_resv_assert_held(): >>>> >>>> WARNING: drivers/dma-buf/dma-resv.c:296 at dma_resv_add_fence+0x71e/0x840 >>>> Call Trace: >>>> virtio_gpu_array_add_fence >>>> virtio_gpu_queue_ctrl_sgs >>>> virtio_gpu_queue_fenced_ctrl_buffer >>>> virtio_gpu_cursor_plane_update >>>> drm_atomic_helper_commit_planes >>>> drm_atomic_helper_commit_tail >>>> commit_tail >>>> drm_atomic_helper_commit >>>> drm_atomic_commit >>>> drm_atomic_helper_update_plane >>>> __setplane_atomic >>>> drm_mode_cursor_universal >>>> drm_mode_cursor_common >>>> drm_mode_cursor_ioctl >>>> drm_ioctl >>>> __x64_sys_ioctl >>>> >>>> Beyond the WARN, mutating the dma_resv fence list without the lock >>>> races with concurrent readers/writers and can corrupt the list. >>> >>> Well why are you trying to add a fence on an atomic mode set in the first >>> place? >>> >>> That is usually an illegal operation here. >> That is pre-existing in the driver. It performs draw operation and in >> some cases waits for the completion during atomic. Whether all that >> syncing is correct is hard to say immediately as some of it may be >> historical edge cases. > > I'm not not so deeply in the atomic mode setting stuff but it strongly sounds > like that this is seriously broken. > > The background is that the atomic mode set framework allows an output > dma_fence which is signaled when the commit is finished. > > So when you allocate a fence slot and add a new fence to finish the atomic > commit it is trivially possible that this cycles back and waits for the > atomic commit to finish. In other words you have a deadlock. > > You probably need specially crafted userspace with the right timing to > trigger that, but such issues are usually a rather big no-no and need to be > fixed in the long term. > > Try to add dma_fence_begin_signaling() and dma_fence_end_signaling() > annotation and enable lockdep, the tool should be able to point out if and > what exactly goes wrong. > > The usual fix is to prepare everything before commit_tail is called (alloc > memory, create, reserve slot, add dma_fence etc....) and then just send out > the prepared commands later on.
We tried with moving resv alloc to prepare_fb() in a previous patch version, it resulted in a non-trivial deadlocks. The goal of this patch is to fix immediate problem with a minimal code change. What you're saying is correct, but it may require a rather big refactoring of the code. In general, everything works okay today, so not really an urgent problem. -- Best regards, Dmitry
